From: http://en.wikipedia.org/wiki/Unix_philosophy

Doug McIlroy, the inventor of Unix pipes and one of the founders of the Unix tradition, summarized the philosophy as follows:

This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.

It's part of the "UNIX philosophy" - to write programs that do one thing, and do it well. They also encouraged the use of text interfaces so that it was easy to connect pieces together with pipes. Text was seen as a "universal" way to transmit data. Such a universal protocol is another concept I think will become more important as the web continues to grow.